Every colour, font and measure on this page is declared in a rule whose selector list also names something PeachPDF has no elements for. Registering those names keeps the list valid, so the half that does match still applies — and the unmatchable half still selects nothing.
:root, :host block — the shape a utility framework emits. Its :root half applies; :host matches nothing.tokensvar()::-webkit-file-upload-button and ::-moz-focus-inner; its panel style from one shared with ::placeholder, and its pills from one shared with :host(.theme).::placeholder:host()